Each piece or length of photographic material treated in a processing tank uses up a certain amount of the processing agent in the tank and as each piece or length of photographic material is taken out of each tank a certain volume of processing solution is taken out of the tank on the surfaces of the photographic material and by-products of processing are deposited in the processing tanks. Thus the strength of the processing solution as well as the volume of processing solution is reduced each time a piece or length of photographic material is processed in the tank. Often the photographic processor comprises a means to monitor the area of photographic material being processed and after a predetermined area has been processed, a solution replenishment cycle is initiated. The frequency of each replenishment cycle is so arranged that the strength of each solution in the processor is returned to the state before processing commenced.
In the past, when replenishing with solution concentrate and water, separate pumps have been used to supply each concentrated processing solution and the water. However, it has been found difficult to accurately adjust the volume of liquid dispensed by each pump during each replenishment cycle to ensure that each tank is replenished to the correct strength.
This is particularly true if the replenishment pumps are bellows pumps wherein problems are experienced associated with variations in the volume dispensed on each stroke of the bellows pump due to the effects of aging and wear of the plastic pump bellows.
Of more concern is the difficulty in adjusting two pump strokes to give adequate degree of accuracy to achieve a desired ratio, typically 1:3. Therefore the majority of systems utilize replenishment from pre-mixed working strength solution, to avoid variations in concentration. But, this is time consuming to prepare such premixed solutions.